Torque T of a synchronous reluctance motor can be expressed by the following equation.T=Pn·(Ld−Lq)·id·iq  (1)
In equation (1), Ld is direct axis inductance, Lq is quadrature axis inductance, id is direct axis current, iq is quadrature axis current, and Pn is the number of pole pairs. Herein, the direct axis inductance Ld is practically determined by the number of armature windings, the gap length, and the magnetic characteristics of the iron core material. As the direct axis inductance Ld is the greatest inductance in a synchronous reluctance motor, the structure of a rotor is determined in such a way that the quadrature axis inductance Lq is as small as possible. In order to reduce the quadrature axis inductance in a state in which the number of windings is constant, it is necessary to reduce the relative permeability, that is, increase the magnetic resistance, at the quadrature axis. Therefore, an existing synchronous reluctance motor is such that the quadrature axis inductance Lq is reduced by increasing the ratio of air per loop of a magnetic flux path along the quadrature axis.
FIG. 5 is a sectional view showing a configuration example of a rotor of an existing synchronous reluctance motor. In the example shown, there are two air layer slits in order to increase the direct axis inductance Ld and reduce the quadrature axis inductance Lq. In FIG. 5, 101 is a rotor iron core, 102 is a first layer slit (outer periphery side), 103 is a second layer slit, 104 is a side bridge (first layer), and 105 is a side bridge (second layer). Herein, it is common that the thicknesses of the side bridges 104 and 105 of the slits 102 and 103 are both the same. Also, the side bridge thickness is preferably as small as possible, as the quadrature axis inductance can be reduced, but as the mechanical resistance with respect to centrifugal force becomes weak, the dimension is determined in accordance with rotation speed.
An advantage of the synchronous reluctance motor is that there is no magnet or the like in the rotor, so the rotor can withstand high speed rotation. However, the synchronous reluctance motor is such that stress concentrates in the side bridge 104 of the slit 102 and the side bridge 105 of the slit 103 shown in FIG. 5 due to centrifugal force working on the rotor. Therefore, the synchronous reluctance motor is such that the dimension of the side bridges needs to be determined so that the stress in the side bridges is restricted to or below the strength of the material thereof.